6.2 Instrument Configuration

FGS

1, 2, 3
(FGS1=FGS1r)

F5ND
F550W
F583W
F605W(FGS1r,FGS3)
F650W (FGS 2)
PUPIL

POS

ACQ-DIST, ACQ-MODE, COUNT, FESTIME, NULL, LOCK

TRANS

ACQ-DIST, ACQ-MODE, SCANS, STEP-SIZE

Pos mode:

F583W

Calibrated across entire FOV

OFAD in F583W only

Pos mode

F5ND

Calibrated at the center and a small number of selected locations in the FOV

Cross-filter calibration with F583W

Trans mode:

F583W, F5ND;
Others as needed

• All calibrations at FOV center
• Select reference S-Curves from library of standard stars
• If needed, propose for calibration time to observe a reference standard star at the same epoch as the science visit (see Chapter 7).

Availability of standard stars with appropriate spectral energy distributions, i.e., colors of reference star and science target should differ by no more than δ(B-V) ~ 0.3 for a given filter (F583W or F5ND).

ACQ-DIST

DEF (default=10),
0.0 to 90.0
 
Units: arcsec

Determines the size (in arcseconds) of the search region. For ACQ-MODE=SEARCH, ACQ-DIST is the maximum radius of the search spiral.
 
Recommendation: Although Search radii up to 90 arcsec are allowed, larger values must be used with caution, i.e., the larger the search radius, the more potential to acquire a spurious nearby star. If the target coordinates are expressed with respect to the Guide Star System, the default value of 10 arcsec (radius) should be sufficient to find the target. Typically the target is found with 0.3 to 1.0 arcsec. Note that large search radii incur a large overhead (time) cost.
 
Reference: Appendix A: Target Acquisition

ACQ-MODE

SEARCH (default)
 
Units: None

Determines the strategy for locating the target. The Search Phase is the initial outward spiral to find and identify the target. The sequence of Search, CoarseTrack, WalkDown, and FineLock is the standard astrometry target acquisition scenario. No other options are available for GO programs.
 
Requirement: Default “SEARCH”
 
Reference: Appendix A: Target Acquisition

COUNT

DEF (default), 1–2621400
 
Units: FGS PMT counts

Expected count rate [target + background]. When the default is set, the count rate is calculated (by the proposal processing software) from the target V magnitude, filter, and aperture. The default background rate (from in-orbit measurements) is 712 counts/sec for all four PMTs.
 
Recommendation: Allow the system to select the default based upon the target’s specified visual magnitude. The default values are calculated using data from on-orbit photometric calibrations. The value of COUNTS is also used to determine the default FESTIME.
 
A few special cases may require non-default values and are discussed in the text.
 
Reference: Chapter 7: See sections on Pos and Trans mode Signal to Noise and Exposure calculation sections.

FESTIME

DEF (default), 0.025, 0.05, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2
 
Units: seconds

Sets the averaging time for the Fine Error Signal (in seconds). The FESTIME also determines the rate at which the star selectors are adjusted to null the fine error signal. The default value for Pos mode is calculated from the COUNT parameter if explicitly entered, or if COUNT defaults to the V magnitude and the filter. All Trans mode observations use FESTIME = 0.025 seconds.
 
Recommendation: The algorithm that selects the default FESTIMEs from the specified brightness of the target is based on photon statistics and a conversion from photon noise to equivalent positional error. For fainter targets, longer integration times are supplied by the default algorithm to assure that the FGS can track the fringe and remain in FineLock.
 
For non-point sources, the S-Curve amplitude is reduced because of the increased contribution of the background or companion. In these cases, it may be desirable to adjust the FESTIME. Please consult with STScI.
 
Reference: Chapter 7: See Sections on Pos and Trans mode Signal to Noise and Exposure calculation sections.

LOCK

FINE (default), COARSE, 0.0–1.0
 
Units: None

The method used by the astrometer to track the science or reference target. (Note, this parameter does not specify the HST guiding mode). Because of the excessive wear to bearings inside the instrument, CoarseLock, which is inferior to FineLock, is no longer allowed.
 
Requirement: Fine.
 
Reference: Chapters 5 and 7.

NULL

YES, NO (default)
 
Units: None

Determines whether the next FESTIME begins immediately after the previous one or not until the star selectors have been repositioned. This parameter may be applicable for very fast moving target. Observers should consult with STScI before specifying non-default values.
 
Recommendation: This use of this parameter is limited to a few specific observation scenarios (tracking solar system objects). Most standard astrometry programs benefit from NULL=NO.
 
Reference: STScI will alert the proposer if a non-default value is required.